This invention relates to a method of evaluating the interfacial strength of a composite material at a boundary between its fiber and matrix. This invention relates also to an apparatus for using such a method.
Composite materials with reinforcing fibers dispersed within matrix are widely in use in various fields of industrial applications such as automobiles, airplanes, tanks and concrete. For evaluating the interfacial strength between the fibers and the matrix of such a material, it is necessary to apply a force on a fiber in order to cause a separation. Since the fibers in such materials are usually very thin with a diameter on the order of 10.about.150 microns, however, use must be made of an indentator with a sharp tip. Since such an indentator penetrates the fiber while pushing it, what is actually being measured is the total displacement of the indentator, including the distance by which the fiber is penetrated by the sharp tip of the indentator. In order to obtain only that portion of the displacement of the indentator at the interface between the fibers and the matrix, therefore, the distance of penetration by the indentator must be subtracted from the measured total displacement.
In order to determine the distance of penetration by an indentator, it has been known to preliminarily provide a correction curve by using a test sample with a very large interfacial strength between its fibers and matrix and to make use of such a curve to make corrections on measured values. It is cumbersome, however, to keep making reference to such a correction curve in order to obtain a desired displacement value at the interfacial boundary. Moreover, there are situations where a sample cannot be manufactured with a satisfactorily large interfacial strength. In such a situation, it is impossible to prepare a correction curve needed for the method. Furthermore, there has not been available any apparatus designed specifically for the measurement of interfacial strength of composite materials. It seems to have been a common practice among some researchers to make changes on a commercially available instrument for measuring hardness, but such a make-shift instrument is not easy to operate, and use was commonly made of an average value of fiber diameters rather than the diameter of the selected fiber which is actually tested. As a result, fluctuations were large, and the conclusions obtained by such prior art methods were not highly reliable.
In view of the above, it is a general object of the present invention to provide an improved method of measuring interfacial strength of a composite material accurately.
It is a more specific object of the present invention to provide such a method which does not requite a sample to be manufactured with a large interfacial strength for preparing a correction curve but is able by a single test to determine the critical value of load at which separation of fibers starts at the interfacial boundary.
It is still another object of the present invention to provide an apparatus which makes use of such a method for the measurement of interfacial strength of a composite material.